Air pollution can have a cooling effect on the climate
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James Hansen, the best known climatologist for alerting the US Congress to global warming in the 1980s, has redoubled its warnings that we underestimate the climate impact of the drop in air pollution.
“Humanity has made a bad deal, a good Faustian deal, when we used aerosols to compensate for almost half of the warming of greenhouse gases,” Hansen told a briefing Hosted by the network of United Nations sustainable development solutions.
But other researchers say that this conclusion is based on trembling foundations, and we still do not know how many reductions in air pollution contribute to global warming. Hansen's conclusions are “oscillating around the upper end of what we will consider as plausible,” says Michael Diamond at Florida State University, which was not involved in research.
Save tips around the world Average temperatures in 2023 and 2024 to have Stimulated debate On the question of whether the pace of global warming accelerates faster than expected. The growing levels of greenhouse gases and an ocean warming the Pacific led most of the temperature increase, but other unknown contributors have pushed even higher temperatures than those explained by these only factors.
Hansen and his colleagues previously linked the acceleration rate of warming with a reduction in air pollution. Now they offer a new analysis arguing that a drop in air pollution can explain the temperature peak in the past two years. Aerosols in air pollution can both directly reflect the sun's sunlight and affect the reflective properties of the clouds- changes in the cloud cover were also involved as a factor of heat.
Researchers focus in particular on the effect of a 2020 Regulation which has reduced the amount of harmful sulfur Used in shipping fuels. This sudden drop in air pollution on the oceans provided researchers with involuntary experience that allows them to determine the climatic effects of aerosols with more precision.
Hansen and his colleagues examined the shipping corridors occupied in the Pacific Ocean to estimate this effect, measuring the change in solar radiation absorbed by the planet in these areas as air pollution has decreased. From that, they believe that the drop in shipping aerosols has increased heat reaching the earth by 0.5 watts per square meter. This is more or less to be warmed from a decade of global carbon dioxide emissions to today's levels.
This additional warming would be sufficient to explain the unexplained part of the heat observed in the past two years, they found. But the implications are wider: this would also mean that the cooling effect of air pollution has been to hide the whole extent of the warming effect of greenhouse gases – in other words, the warming experienced to date does not represent the full impact of our emissions.
Hansen and his colleagues warn that this means that the climate is much more sensitive than expected to increase greenhouse gas levels. Consequently, they argue, the world approaches climate change points more quickly, such as the slowdown in the key Currents of the Atlantic Ocean and the collapse of the glacial cap of Western Antarctica. To fight against this, they say that we should more seriously consider how to cool the planet with interventions like solar geo-engineering.
However, the number of square meters of 0.5 watts at the heart of the new analysis is much higher than other estimates of the warming effect of change in shipping emissions, says Tianle Yuan At the University of Maryland Baltimore County. But he says it is not completely improbable.
Gavin Schmidt At NASA, NASA claims that the number is “most likely an overestimation” because it assumes that all the change in the sunlight absorbed is due to the change of shipping aerosols, rather than other changes such as less pollution of China or natural variability.
A change in aerosols may even not be necessary to explain the temperature peak of 2023, says Shiv Priyam Raghuraman at the University of Illinois Urbana -Champaign – he previously found that it could be explained by Changes in Pacific Ocean temperatures alone. He says that more work is necessary to reconcile various estimates of the warming effects of aerosols.
